1. Field of the Invention
The present invention concerns a piezoelectric transducer and, more particularly, a transducer used to generate bulk waves in the microwave range, of the order of one gigahertz, in a crystalline substrate constituting a support. The generation of bulk waves enables the making of a bulk wave delay line, or an acousto-optical transducer, called a Bragg cell, to diffract and modulate this incident laser beam.
2. Description of the Prior Art
FIG. 1 appended hereto gives a schematic view of a bulk wave line comprising a substrate SC, such as a bar of corundum, and a piezoelectric transducer supported by a plane face FS of the substrate. A first electrode E1 of the transducer is formed by a ground contact plane which is metallized throughout the substrate face FS. A thin piezoelectric plate LP with a high coupling coefficient is centered on the metallized plane electrode E1. At the center of this plate, a second electrode E2 is deposited. The electrodes E1 and E2 are connected to a microwave signal generator GH, such as a reception antenna amplification circuit, for example by means of an impedance matching microstrip microwave line on ceramic substrate, to produce microwave acoustic waves OA that are propagated longitudinally in the substrate SC and perpendicularly to the face FS of this substrate.
For reasons of technological ease and dependability, the plate LP is appreciably bigger than the second electrode E2, which should be small to reduce the electrical capacitance of the piezoelectric transducer and thus increase the efficiency of this transducer. However, the admittance of the transducer cannot be reduced to the extent desired, because of the solder spot or "ball" covering the second electrode E2, necessary to connect it to the generator GH. For, the "ball" of solder has a diameter equal to at least 50 .mu.m and the dimensions of the electrode E2 therefore cannot be smaller than this dimension.
Furthermore, through its weight, the "ball" of solder causes non-reproducible disturbances in the generation of the acoustic waves and offers a relatively high degree of series self-inductance.
During the soldering operation, there is the risk that the electrode E2 as well as the plate LP may be damaged.
The present invention is therefore aimed at overcoming mainly the drawbacks inherent in the direct connection, by soldering, of the second electrode, in moving over the connection of this electrode so as to make its dimension independent of the size of the solder. A smaller effective contact region of the second electrode is obtained, enabling the admittance of the piezoelectric transducer to be reduced.